The invention relates to an arrangement for demodulating a frequency-modulated input signal by means of a filter which, in the range of the possible frequencies of the input signal, has a sloping frequency-amplitude characteristic.
Arrangements of this type are required to convert a frequency-modulated signal in the baseband, that is to say to recover the original information. Frequency-modulated signals are used in, for example, color television techniques, inter alia for the two chrominance signals in the SECAM-method and in magnetic picture recording, the luminance signal being converted into a frequency-modulated signal and being recorded in this form on the magnetic tape. This usually relates to what is commonly referred to as broadband frequency modulation, in which the maximum frequency swing is of the order of magnitude of the center frequency of the frequency-modulated signal.
An arrangement of the type described in the opening paragraph is known, for example from "Elektronik-Arbeitsblatter", Franzis-Verlag, Munich. In these prior art arrangements, the filter is constructed by means of coils and capacitors, for example, for use as a resonant circuit. Such circuits are indeed relatively simple but the results obtained therewith cannot satisfy more stringent requirements; more specifically an arrangement having a resonant circuit is not suitable for broadband frequency modulation. Difficulties are also encountered when the usual signal in the baseband, that is to say the modulating frequency, extends to near the center frequency of the frequency-modulated signal. Quite a number of other circuits for frequency demodulation are known, for example push-pull discriminators or ratio detectors, which indeed produce very satisfactory results, particularly at narrow-band frequency modulation, but they all use coils and capacitors and consequently cannot be manufactured in integrated circuit technology.
In substantially all the known frequency demodulator circuits, a limiting amplifier precedes the frequency demodulator circuits, which limiting amplifier has for its object to prevent the demodulated signal from being affected by amplitude demodulation of the frequency-modulated signal as a result of disturbances. This limiting function can also be given to the demodulator itself, for example to the ratio detector.